High impedance state for digital subscriber line transceivers on copper twisted pairs

ABSTRACT

The invention relates to methods and a high speed communication device that allow one of a plurality of high speed communication devices connected to a transmission line having a normal impedance to effectively receive data. 
     The high speed communication device can include a transmission line interface connected to the transmission line, a receiver connected to the transmission line interface, and a transmitter selectively coupled to the transmission line interface. The transmitter can have an impedance substantially equal to the normal line impedance. 
     The high speed communication device can present a high impedance to the transmission line with respect to the normal line impedance when the transmitter is not coupled to the transmission line interface, and the high speed communication device can present an impedance to the transmission line that is substantially equal to the normal line impedance when the transmitter is coupled to the transmission line interface.

FIELD OF THE INVENTION

The invention relates to high speed communication devices for use as oneof a plurality of high speed communication devices connected to atransmission line having a normal or characteristic impedance.

BACKGROUND OF THE INVENTION

Communications transceivers such as telephones, fax machines and voicemodems, that are designed to work on copper twisted pair wirecommunication lines and transmit signals in the 200 Hz to 4 kHzfrequency range support the connection of more than one transceiver at aremote customer end of the line. This is generally possible through thedefinition of two states of such equipment called the On-hook andOff-hook states. These are derived from conventional telephony termsthat indicate whether the handset of the telephone is on or off hook.Equipment in the Off-hook state is active and participates in acommunication session. Equipment in the On-hook state is inactive anddoes not participate in a communication session, has negligible impacton the line, and can be ignored by other equipment connected to theline. Typically, at any given instant, only one piece of equipmentconnected to the line is in the Off-hook state, while others are in theOn-hook state.

Although work on DSL transceivers dates back to the latenineteen-eighties and early nineteen-nineties, present DigitalSubscriber Line (DSL) transceivers and other high speed communicationdevices are not designed to support the connection of more than onetransceiver or transmitter at any given end of the line. Additional highspeed communication devices, or DSL transceivers, which is one type ofhigh speed communication device, cannot be connected to either end ofthe line without incurring a significant loss in the quality oftransmission and reception of signals.

Examples of high speed communication systems, such as DSL communicationssystems, are Asymmetric DSL (ADSL) and Very high speed DSL (VDSL). xDSLequipment can generally be broken down into two basic units, the xDSLTransceiver Unit Central Office (xTU-C) and xDSL Transceiver Unit Remote(xTU-R).

In theory, a copper twisted pair of wires of infinite length has acharacteristic impedance for a given frequency. In practice, however,the copper twisted pair of wires has a finite length and a normalimpedance. The normal impedance typically has a slightly lower impedancethan the characteristic impedance but is substantially equal to thecharacteristic impedance. In this disclosure, the terms normal impedanceand characteristic impedance are used interchangeably.

DSL transceivers in use are only capable of presenting an impedance thatis substantially equal to the characteristic impedance of the coppertwisted pair of wires over the transceivers' frequency range ofoperation. This impedance is generally 100 Ohms. Ordinarily, thisensures that echo is kept at a minimum and maximum power transfer iseffected into or from the line, as shown in FIG. 1. This result does notfollow if more than one transceiver is connected at any end, inparallel.

Several problems arise if more than one DSL transceiver presently in useis connected at one end of the line. For example, problems related toattenuation, echo, contention, and non-deterministic impedance whenpowered-off may arise. This is because, typically, the additionaltransceiver or transceivers offer the same impedance, i.e. the normalimpedance, across the line.

First, the additional transceiver(s) causes signal power attenuation.Specifically, the power of the signal received from the other end of theline is divided into each of the transceivers at the end where more thanone transceiver is connected (parallely connected transceivers). At theparallely connected end, as shown in FIG. 2, each transceiver receivesless power than it would have received if it were the only one connectedat that end.

FIG. 6 summarizes simulations that have been performed on SPICE tomeasure the impedance presented to the line when two ATU-Rs areconnected in parallel. As shown in FIG. 6, the power of a signalreceived by an ATU-R is reduced by about 4.8 dB if another ATU-R, whichalso has a termination of 100 Ohms, is connected in parallel. Here theloop has not been included. Depending on the loop length, the powerreduction can be up to 6 dB for one additional ATU-R.

As shown in FIG. 7, the impedance presented to the line at the parallelyconnected end (the effective line impedance) is further reduced whenmore than one ATU-R, each terminated with the normal impedance of 100Ohms, is connected in parallel. Because the line is now terminated witha lower impedance than the characteristic or normal line impedance, thesignal power transfer from the transmitter at the other end of the lineinto the line will be less than the possible maximum (as per the MaximumPower Transfer Theorem: Schaum's Outline Series Theories And Problems OfElectric Circuits By Joseph A Edminister, Published By McGraw Hill BookCompany (August 1965). This reduced power then travels along the lineand further gets distributed amongst the ATU-Rs connected at the end ofthe line as mentioned above.

Further, as shown in FIG. 3, each transceiver connected in parallel to atransmitting transceiver acts as a load on signals transmitted by thetransmitting transceiver. Hence, the total power transmitted onto theline is less than the power that would have been transmitted if only onetransceiver was connected at the transmitting end of the line. Thetransceiver at the other end of the line (distant-receivingtransceiver), therefore, receives signals having significantly reducedpower than the distant-receiving transceiver would have received if asingle transceiver were connected at the transmitting end of the line.

Second, the connection of additional transceivers in parallel alsoresults in increased echo. Presently, transceivers are designed totransmit onto a line terminated at the other end by the characteristicimpedance. When more than one transceiver is connected to a transmittingend of the line in parallel, as depicted in FIG. 4, the impedancepresented to the transmission line by all the transceivers connected inparallel at the transmitting end of the line is much less than thecharacteristic or normal impedance. This will result in echo at thetransmitting end of the line.

Echo, at any interface, is dependent on line termination impedance, andgenerally the effectiveness of echo suppression and cancellationcircuits is dependent on the termination presented by the interface.Connection of more than one transceiver on the line changes thistermination and thus increases the echo. FIG. 8 summarizes the resultsof a simulation and shows the drastic increase in the echo seen by thetransmitting ATU-R when additional ATU-Rs with a termination of 100 Ohmsare connected in parallel.

As shown in FIG. 5, additional transceivers connected in parallel at areceiving end will also present impedance that is less than thecharacteristic line impedance to signals arriving at the receiving endof the line. This will cause a larger reflection of the signal back intothe line resulting in a larger echo at the interface ‘R’, resulting inan increased echo at the distant-transmitting transceiver.

The transceivers will hence be subject to higher echo, causingcomplications associated with increased echo in the performance of thetransceivers. Echo, if not cancelled or suppressed, would reduce theeffective dynamic range of the receiver and could, in a worst casescenario, saturate the receiver altogether. Reduction of the dynamicrange could adversely affect the performance of the receiver resultingin lower bit rates and saturation could cause non-linear distortion alsoresulting in lower bit rates.

Simulations have been performed on MATLAB, for a specific test case(Case#3 in Annex D of the ITU-T G.992.2 standard), which involves theloop T1.601#7 (13.5 kft plain loop) and a 24 DSL NEXT (Near Endcrosstalk) and a −140 dBm/Hz background noise (ITU-T RecommendationG.992.1 (06/99), “Asymmetric Digital Subscriber Line (ADSL)Transceivers”, International Telecommunication Union). One active ATU-Rmodem communicating with the ATU-C and a number of passive ATU-Rsconnected at the remote end have been simulated. The SNR profile issimulated for one ATU-R and for each additional passive ATU-R (max offour). The results are shown in FIG. 12. From this figure we see thatthe SNR profile drops drastically, even if one additional ATU-R isadded.

The following table shows the bit rates generated for the above SNRprofiles, after giving 4 dB margins.

Number of Additional ATU-Rs Bit Rates (kbps) 0 2208 1 1440 2 1120 3 9284 800

Third, when more than one transceiver has to be connected on the samemedium, contention problems arise. Contention is the inadvertent,simultaneous transmission of similar signals by more than onetransmitter onto the same transmission medium, at the same end inparticular. This could result in signal corruption.

It has been suggested, in line with the prevalent method of avoidingcontention in data communication systems, that the transceivers couldcheck or sense the medium to determine whether the medium is idle beforeactually transmitting signals. This is possible if the receiver canprobe for signals on the line without loading existing signals, if any,on the line to avoid corrupting existing signals and to preventdisrupting communication in progress between other transceivers.However, if a transceiver that has a normal impedance probes the line,it will also load the line. Therefore, it is desirable for a receiver topresent a high impedance across the line to probe. Existing DSLtransceivers are not capable of presenting a high impedance across theline while receiving the signal and hence could disrupt on-goingcommunication.

Fourth, the impedance of existing DSL transceivers, when powered off, isnot known. This is because the network that constitutes the impedanceseen by the line may contain active devices, and the characteristics ofactive devices cannot be determined in the power-off condition.Therefore, if a current DSL transceiver remains connected to the linewhen powered off, it could be presenting a low impedance across the lineand hence, any of the other transceivers connected to the same line arenot guaranteed to work at their rated performance level or may not workat all. Even if transceivers had a mechanism incorporated to ensureautomatic line disconnection when powered off, they would immediatelyget connected to the line and offer normal impedance when switched on,and could thus potentially disturb ongoing communication on the line.The user would have to ensure that other modems connected to the lineare not communicating before switching the modem on. Therefore themechanism stated above, although automatic in one sense, would notactually serve its purpose.

Given the problems that arise when more than one DSL transceiver,presently in use, is connected across the same line, it has beensuggested that the only way to resolve the aforementioned problems is tocheck and ensure that all other transceivers are disconnected from theline before using any one transceiver on the line. All users would haveto carry out this cumbersome procedure whenever they want to use theirDSL modem. As explained above, even powering off the other transceiversmay not help. In practice it would render the use of multiple DSLtransceivers on the same line very unwieldy as compared to the use ofvoice band transceivers, the predecessors of the DSL transceivers. Itshould be noted that although the above discussion refers to DSLtransceivers, the problems noted above apply to other high speedcommunication devices as well, such as high speed communicationtransmitters and co-axial modems for example. Accordingly, therecontinues to be a need for a high speed communication device thatresolves the problems mentioned above.

BRIEF SUMMARY OF THE INVENTION

The invention relates to high speed communication devices configured toallow a plurality of high speed communication devices to be connected toan end of a transmission line having a characteristic or normalimpedance and effectively receive data through high and normal impedancestates.

A high speed communication device is provided. The high speedcommunication device includes a first receiver, a line interface, whichmay be a hybrid circuit, and a transmitter selectively coupled to thetransmission line. In a high impedance state, the transmitter is notcoupled to the transmission line and the high speed communication devicepresents a high impedance to the transmission line with respect to thenormal impedance of the transmission line. In a normal impedance state,the transmitter is coupled to the transmission line and the high speedcommunication device presents an impedance to the transmission linesubstantially equal to the normal impedance of the transmission line.

The high speed communication device is open to several embodiments. Forexample, the transmitter may be selectively coupled to the hybridcircuit by a first switch and the hybrid circuit may be connected to thetransmission line in a first embodiment. In a second embodiment, thefirst switch may be positioned within the transmitter.

In a third embodiment, the transmitter may be connected to the hybridcircuit, which can be selectively coupled to the transmission line by afirst switch. The receiver may be selectively coupled to the hybridcircuit by a second switch.

In a fourth embodiment, the high speed communication device may furtherinclude a second receiver. The transmitter can be connected to thehybrid circuit, which can be selectively coupled to the transmissionline by a first switch. The first receiver can be connected to thehybrid circuit, and the second receiver can be selectively coupled tothe transmission line by the first switch.

Pursuant to a first aspect of the invention, high speed communicationdevices in the high impedance state present a high impedance withrespect to the characteristic or normal line impedance, do not transmitdata, but can receive data. Pursuant to a second aspect of theinvention, one of the plurality of devices is in the normal impedancestate when it presents an impedance substantially equal to thecharacteristic or normal line impedance, and is capable of transmittingdata, at all times. Pursuant to the second aspect, the device may switchout of or into the normal impedance state, and another of the pluralityof devices may switch into or out of the normal impedance state.

A system is also provided that includes at least one central office highspeed communication device connected to a first end of a transmissionline having a normal impedance, and a plurality of remote high speedcommunication devices connected to a second end of the transmissionline. The plurality of remote high speed communication devices can becapable of switching from an active state, when they can transmit andreceive data across the transmission line, and a high impedance state,when they can probe and sense the transmission line for data. Each ofthe plurality of remote high speed communication devices can have adifferent configuration. For example, one of the plurality of high speedcommunication devices may take the form of the first embodimentdescribed above, while another of the plurality of communication devicesmay take the form of the third embodiment described above.

Pursuant to aspects of the invention, protocols may dictate when one ofthe plurality of high speed communication devices can transmit data orincorporate data that indicates an intended device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a transceiver in use connected to a transmission line;

FIG. 2 depicts the power attenuation of received signals when sent to aplurality of transceivers in use connected in parallel to a transmissionline;

FIG. 3 depicts the power attenuation of transmitted signals when sent bya plurality of transceivers in use connected in parallel to atransmission line;

FIG. 4 depicts the echo of transmitted signals when sent by a pluralityof transceivers in use connected in parallel to a transmission line;

FIG. 5 depicts the echo of received signals when sent to a plurality oftransceivers in use connected in parallel to a transmission line;

FIG. 6 is a graph that depicts the reduction in a signal received by aremote high speed communication transceiver when two high speedcommunication transceivers are connected in parallel;

FIG. 7 is a graph that depicts the reduction in the effective impedancewhen a plurality of high speed communication transceivers are connectedto an end of a transmission line;

FIG. 8 is a graph that depicts the increase in echo at a remote highspeed communication transceiver that results from other remote highspeed communication transceivers;

FIG. 9 depicts a first embodiment of a high speed communicationtransceiver incorporating a receiver, a transmitter, a hybrid, and aswitch to effectuate high and normal impedance states pursuant toaspects of the invention;

FIG. 10 depicts a third embodiment of a high speed communicationtransceiver incorporating a receiver, a transmitter, a hybrid, and twoswitches to effectuate high and normal impedance states pursuant toaspects of the invention;

FIG. 11 depicts a fourth embodiment of a high speed communicationtransceiver incorporating a first receiver, a second receiver, atransmitter, a hybrid, and two switches to effectuate high and normalimpedance states pursuant to aspects of the invention;

FIG. 12 is a graph that depicts simulation results of SNR profiles formultiple high speed communication transceivers; and

FIG. 13 depicts a detailed implementation of a high speed communicationtransceiver including a line driver (transmitter), a line receiver(receiver), a probing transformer (protection circuit), a linetransformer (hybrid), and two electro-mechanical switches to effectuatehigh and normal impedance states pursuant to aspects of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following specification describes specific methods and embodimentsof the invention with the understanding that the scope of the inventionis not to be limited to the specific methods and embodiments depictedbelow. Those skilled in the art will recognize that several othermethods and embodiments, not described below, still fall within thescope and spirit of the invention.

Pursuant to one aspect of the invention, problems encountered whenconnecting high speed communication devices in parallel are addressed bydefining a state for the parallely connected devices, the ‘HighImpedance’ (HI) state of a high speed communication device.

High speed communication devices in the HI State, present a highimpedance across the line with respect to the characteristic or normalline impedance, do not transmit signals, and can receive signals fromother transceivers transmitting on the same line.

Pursuant to a second aspect of the invention, one of the parallelyconnected high speed communication devices at an end of the line, is ina normal impedance (“NI”) state, at any given time. For example, thehigh speed communication device in the NI state could exchange messageswith a high speed communication device at the other end of the twistedpair line, while the remaining parallely connected high speedcommunication devices on the transmitting end stay in the HI state. Thehigh speed communication device in the NI state would thus be active, asdescribed below. Another high speed communication device at theaforementioned parallely connected end can switch to the NI state andthe previous high speed communication device in the NI state wouldchange to the HI state, as described below.

Those of ordinary skill in the art will recognize that there may bedifferent types of high speed communication devices connected to thesame line, such as DSL transceiver modems (ADSL, HDSL, VDSL, etc),co-axial modems, or high speed communication transmitters. Consequently,the frequency spectrum of usage could be different. Because the CI ofthe line is different at different frequency ranges, the value of the HIshould be defined to address the highest of the CIs, unless otherprecautions are taken, such as, for example, the use of POTS splitters,to isolate the DSL transceivers from each other. The POTS splitterisolates the ADSL transceivers on the line from the POTS equipment andthus the low impedance offered by the POTS equipment in the Off HookState would not affect the impedance of the line at the ADSLfrequencies. In the discussion below, high speed communicationtransceivers, and in particular DSL transceiver modems, are discussed indetail. However, those of ordinary skill in the art will recognize thatmuch of the discussion also applies to other high speed communicationdevices, such as, for example, high speed communication transmitters.

Those of ordinary skill in the art will recognize that, the impedancevalue Z_(HI) in the HI state could also depend on the number oftransceivers in the HI state that can be connected (per systemrequirements) in parallel, to one transceiver in the NI state. That is,if the number of parallely connected transceivers is ‘n’, the effectiveparallel combination of the ‘n’ high impedances should not affect thesignal on the line. Therefore:Z _(HI) /n>>Z _(NI) or Z _(HI) >>Z _(NI) *n

In general, the impedance of the parallely connected transceivers in theHI state should be high enough, with respect to the CI of the line inthe frequency range of operation of all the transceivers, to ensure thatsignal loading on the line by the transceivers in the HI state isnegligible. The ideal HI value is infinite.

TABLE 1 Transceiver State Impedance UNPOWERED Z_(HI) Disabled Z_(HI)(Powered with transmitter and receiver inactive) Inactive Z_(HI)(Powered with transmitter inactive and receiver active to detectsignals) Active Z_(N) (powered with transmitter and receiver active andinitializing or in showtime)

Table 1 summarizes the impedance transceivers should present across theline at different transceiver power states, wherein Z_(HI) is theimpedance in the High Impedance state and Z_(N) is the impedance in theNormal Impedance State.

In practice, while more than one DSL transceiver can be parallelyconnected to an end of the line, at any given point of time, only one ofthese DSL transceivers can transmit data across the line. The followingsummarizes one possible state transition scheme pursuant to aspects ofthe invention. To start, assume that all parallely connectedtransceivers at a second end of the line are kept in the HI state. Twocases are possible:

In a first case, the transceiver at a first end of the line mightinitiate a transaction by transmitting a predefined signal. In thiscase, each parallely connected transceivers at the second end willreceive the initiation signal and only the transceiver to which thesignal is intended will change over to the NI state and respond with thetransmission of an appropriate signal(s). Those skilled in the art willrecognize that several protocols known in the art are capable ofindicating the intended transceiver.

In a second case, one of the parallely connected transceivers at thesecond end might initiate a transaction based on a protocol. Theprotocol may be based on time slots or line sensing. For example, for aline sensing based protocol, the transmitting modem can check if theline is idle (no signal) for a pre-defined duration of time while in theHI State, and if the line is idle, the modem can change over to the NIstate and start transmitting a signal. The transmitting modem would knowthat messages arriving from the first end of the line are directedtowards it. Two or more transceivers may start transmitting at the sametime and cause a collision. Those skilled in the art will recognize thatthere are protocols to detect and resolve such collisions.

In either case, transceivers not transmitting signals and to whichsignals are not intended, can wait until the intended transceivers andtransmitting transceivers stop transmitting, and wait until the linebecomes idle for a pre-defined minimum duration. Therefore, at a lineend, only one parallely connected DSL transceiver that transmits shallcontinue to be in, or change over to, the NI state for transmissionpurposes. After transmission is completed, it can revert to the HIstate, or remain in the NI state.

It should be noted that if each transceiver connected to one end of along line remains in the HI state, as assumed in the earlier paragraph,then the line will be effectively unterminated at that end, meaning animpedance mismatch would result. Consequently, signals transmitted bytransceivers at the other end of the line may not be received properlyby the parallely connected transceivers in the HI state because ofreflections at the unterminated end. Although transceivers in the HIstate are capable of receiving signals on the line, the signal mightoccasionally be sufficiently distorted from reflections caused by theunterminated end to prevent proper reception.

There are several methods to address the unterminated end problemdescribed above. For example, in one method, one transceiver at eitherend of the line can remain in the NI state, even during the idle statesof the line. The impedance presented by this transceiver can thenprovide sufficient line termination to prevent the signal from the otherend of the line from being reflected by the impedance mismatch at theunterminated line end. Therefore, other transceivers connected at thesame end would receive signals properly, even though they would be inthe HI state. After determining the intended transceiver to which thetransmission is directed, that intended transceiver will either continueto remain in, or changeover to the NI state, and the other parallelyconnected transceivers will either continue to remain in, or changeoverto the HI state.

In another example, all the transceivers connected in parallel at oneend of a transmission line could be in the HI state during the idlestate of the line, provided an incorporated communication protocolincludes an initiation phase (For example, the hand shake phase definedin ITU-T Recommendation G.994.1 (06/99), “Hand Shake Procedures for DSLTransceivers”, International Telecommunication Union). The initiationphase can be designed to be sufficiently robust to receive signals anddetermine an intended transceiver despite problems arising from improperline termination and signal distortion. The intended transceiver couldchange over to the NI state and the other transceivers could remain inthe HI state.

The two DSL transceivers that communicate with each other may beconnected in various manners, for example they may each be at separateends of the transmission line, at the same end of the transmission line,or at the end of bridge taps.

Bridged taps are unused twisted-pair cables attached to a telephonesubscriber loop. For example, bridged taps are additional, unusedtwisted pairs of wires that are connected to the main subscriber lineused for connecting telephones to a central office. The bridged tapscould have been lines that were previously used by other subscribers whosubsequently surrendered their lines. The service provider either didnot disconnect the lines, or left them as is, hoping to use them laterand extended the line for the use of another subscriber. Typically,these bridged taps have very little or no affect on signals at low(POTS) frequencies, and hence do not impact the use of telephones.Bridged taps, however, have significant effect on signals of higherfrequencies.

Typically, bridged taps should not be terminated in order to allow theDSL systems to function properly. Consequently, it was not possible toconnect DSL transceivers that only presented a normal impedance at theends of bridged taps. Pursuant to aspects of the invention, DSLtransceivers in the HI state also facilitate connecting DSL transceiversat bridged tap ends in addition to regular line ends. Like the scenariosdiscussed above, at any given time, only one DSL transceiver should bein the NI mode while the rest are in the HI state for bridged tap endconnections.

The aspects of the invention address several problems discussed above.For example, the invention ensures that the quality of signals receivedby a plurality of transceivers connected to a second end of a line wouldbe the same as if the signals were received by a single transceiverconnected to the second end of the line.

When transmitting signals using transceivers that incorporate aspects ofthe invention, the transmitted signal will not be loaded by othertransceivers connected in parallel to the transmitting transceiver at afirst end of the transmission line. This ensures that the powertransmitted by the transceiver will be transmitted onto the line. Hence,the power received by a transceiver connected at a distant-receiving endof the line would be the same as if the transmitting transceiver werethe only one connected to the first end.

The signal echo problem caused by a plurality of digital communicationtransceivers is also addressed by the invention. Pursuant to the secondaspect of the invention, one receiving transceiver on a second end ofthe line would be in the NI state while other parallely connectedreceiving transceivers at the second end would be in the HI state.Therefore, the effective impedance presented by the parallely connectedreceiving transceivers would be substantially equal to the normal lineimpedance. Consequently, the echo that a transmitting transceiver at thefirst end is subject to from the parallely connected transceivers at thesecond end would be substantially equal to the echo received if only onereceiving transceiver were connected at the second end.

Those of ordinary skill in the art recognize that DSL transmitters arevoltage sources and that DSL receivers are voltage sensors. Therefore,basic circuit law shows that if a transceiver that is configured todrive a signal onto a medium of impedance Z_(N) has a source impedanceof Z_(HI), where Z_(HI) is >>Z_(N), then the signal power actuallytransmitted onto the line would be small, i.e., Z_(N)/(Z_(N)+Z_(HI))times the signal power actually outputted by the transceiver. Therefore,it is difficult for a transceiver in the HI state to transmit a signalof sufficient power onto a medium. To the contrary, a receiver connectedin parallel to the transmitter can still receive signals whilepresenting a high impedance across the line. The receiver, therefore,does not load the line because of the high impedance. Therefore the HIstate, would enable a parallely connected transceiver to receive and/orprobe the line for signals while not loading the signals, if any, on theline and thus check if the line is in use, before initiating atransmission.

The problem caused by indeterminate impedance of a transceiver when itis powered-off is resolved by ensuring that transceivers are in the HIstate when they are powered-off. Thus, the transceivers do not have tobe disconnected from the line.

On the other hand, when a modem is switched on, and the transceiverchanges from the ‘Powered off’ state to the ‘Inactive’ state, as shownin table 1 above, the invention allows the transceiver to continue inthe HI state and probe the line for signals without distorting them,before it starts transmission.

Those skilled in the art will recognize that the HI and NI states fordigital communication transceivers, as described above, may beeffectuated in several forms and embodiments. Specific embodiments aredescribed below and in the drawings with the understanding that thedisclosure is to be considered an exemplification of the invention andis not intended to limit the invention to the specific embodimentsillustrated in this disclosure.

Typically, digital transceiver modems include a transmission lineinterface that comprises a two-to-four wire converter called the hybrid.The term two wire is used to describe the duplex port of the line andthe term four wire is used to describe the simplex transmit and receiveports, each being two terminal. The hybrid could be transformer-based ortransformer-less. The hybrid reflects the impedances from the two wireto the four wire ports and vice versa with certain multiplicationfactors that are dictated by hybrid characteristics (turns ratios fortransformer-based hybrids).

Digital transceiver modems typically include a transmitter Tx and areceiver Rx. Those of ordinary skill in the art recognize that thetransmitter Tx and receiver Rx are generally coupled to other parts ofthe digital transceiver, such as an analog front end AFE. A sourceresistance (internal plus external) of the transmitter Tx (alsocharacterized as the line driver) can also serve as a terminationresistance for signals received from another end of the line. The sourceresistance is generally designed to be approximately equal to thecharacteristic or normal impedance of the medium, such as a twistedpair, over the considered frequency band, in accordance with the maximumPower Transfer Theorem, as explained above. This addresses impedancereflection ratios. The receiver Rx of the transceiver is generallydesigned to have high input impedance.

Embodiments of the invention described below incorporate one or twoswitches that are used to set the NI or the HI state of the transceiverwhile enabling the transceiver to receive signals from the line. Thefollowing illustrations are simplified for ease of comprehension asfollows: (1) Although realization is generally differential, thefollowing illustrations are single ended; and (2) Echo cancellation andEcho suppression circuitry are eliminated. Those skilled in the art willrecognize that each of the following illustrations can easily beconverted into a differential version for actual implementation. Adetailed implementation of the third embodiment is described below.

FIG. 9 shows a first embodiment that incorporates aspects of theinvention. FIG. 9 illustrates a transmitter Tx, a receiver Rx and ahybrid circuit. A single pole single throw (SPST) switch Sw1 can be usedto selectively couple the transmitter Tx to the hybrid. The receiver Rxmay be continuously connected to the hybrid. When the switch Sw1 isclosed, the transmitter Tx is coupled to the hybrid, which is coupled tothe transmission line, and the transceiver is in the NI state. Theinternal and external resistance of the transmitter Tx can besubstantially equal to the characteristic or normal line impedance.Consequently, in the NI state, the impedance seen by signals transmittedfrom the other end of the line is substantially equal to thecharacteristic or normal line impedance modified by the hybridreflection ratio. In the first embodiment, when the switch Sw1 is open,the transmitter Tx is not coupled to the hybrid and the transceiver isin the HI state. The source resistance in the HI state, which isdictated by the receiver resistance, then, is very high (theoreticallyinfinite) as the path to ground through the transmitter is switched off.Consequently, the impedance seen by the line, and presented by thetransceiver, would also be high.

Pursuant to a second embodiment of the invention, the transmitter Tx maybe designed to present a high impedance at the output terminal(s)through a control method or configuration. In the second embodiment, theswitch Sw1 may not appear as an explicit block schematic outside of thetransmitter Tx, but within the transmitter Tx (internally). The systemcould then use this feature to set the transceiver in the HI mode in thesame way as described in the first embodiment.

Pursuant to a third embodiment of the invention, shown in FIG. 10, thehybrid is selectively coupled to the transmission line by a third switchSw3. Those skilled in the art will recognize that the third switch Sw3can include first, second and third terminals 1, 2 and 3 with the firstterminal 1 being the common terminal, and the second switch Sw2 caninclude fourth, fifth and sixth terminals 4, 5 and 6 with the fourthterminal 4 being the common terminal. In the third embodiment, thereceiver Rx is coupled to the transmission line in the HI State throughtwo single pole double throw switches Sw2 and Sw3. As shown in FIG. 10,the transmitter Tx and hybrid are not coupled to the transmission linein the HI state since the transceiver does not transmit signals in theHI State. In the HI state, the common terminal of the third switch 1 isconnected to the third terminal of the third switch 3 while the commonterminal of the second switch 4 is connected to the fifth terminal ofthe second switch 5.

In the NI state of the third embodiment, the transmission line iscoupled to the hybrid by the third switch Sw3 and the common terminal ofthe third switch 1 is connected to the second terminal of the thirdswitch 2 while the common terminal of the second switch 4 is connectedto the sixth terminal of the second switch 6.

In an alternate embodiment of the third embodiment, a protection circuitshown in FIG. 10, is coupled to the receiver in the HI state. Desirably,the protection circuit protects the receiver from line surge voltages.

In a fourth embodiment of the invention, a second receiver Rx2 andfourth and fifth switches Sw4, Sw5 are incorporated. The behavior of thereceiver in the first, second and third embodiments may be affected fromthe absence of the termination offered by the line driver (transmitter),particularly with respect to the echo suppression function.Consequently, it may be difficult to introduce switches as described inthe first three embodiments without affecting the behavior of thereceiver Rx1 in the HI State. In other words, it may be difficult to usethe receiver Rx1 or each part of the same receiver Rx1, which wasdesigned for the NI state, effectively in the HI state. Therefore, itmight be beneficial to provide the second receiver Rx2, or parts of it,specifically designed for the HI state. The transmission line can thenbe decoupled from the hybrid and coupled to the second receiver Rx2 inthe IH state by using the fifth switches Sw5, as depicted in FIG. 11. Inthe NI state the first receiver Rx1 can be coupled to the transmissionline by the fifth switch and the hybrid. Also, as shown in FIG. 11, thefourth switch Sw4 can couple the first receiver to other parts of thetransceiver, such as the analog front end AFE, in the NI state, and cancouple the second receiver to other parts of the transceiver, such asthe analog front end AFE, in the HI state. In an alternate embodiment ofthe fourth embodiment, a protection circuit is desirably coupled to thesecond receiver, as shown in FIG. 11.

TABLE 2 Relay conditions for Impedance States of the Transceiver RelaysTransceiver K1 K2 State Function Operated Operated Normal Transmit andImpedance Receive State Operated Released Invalid Invalid ReleasedOperated Invalid Invalid Released Released High Line Probe ImpedanceState

The following is a detailed description of the third embodiment of theinvention. Those skilled in the art will recognize that the thirdembodiment described above is sufficient to make and use the thirdembodiment of the invention without undue experimentation.

FIG. 13 shows a detailed implementation of the High impedance state ofan ADSL Modem transceiver. In the detailed implementation, theelectromechanical relays R1, R2 together are used to change the state ofthe transceiver between High and Normal Impedance States. The relays canbe controlled through software and/or hardware. The Probing transformerand its associated circuitry are also additional elements to implementthe HI state. The rest of the circuitry is typically found in regulartransceivers.

In the detailed implementation, when both relays R1, R2 are operated, asindicated in table 2, the transceiver will be in the Normal ImpedanceState. The transmission line, indicated by connectors 9A and 4A, isconnected to the line transformer through Relay K1 and further to theline driver (transmitter). The line receiver (indicated in small print)is also connected to the line transformer through Relay K2. The ProbingTransformer and its associated circuitry are isolated from the linethrough K1 and from the line receiver through K2. Resistors R28 and R48are each equal to half the characteristic or normal impedance of theline (without bridged taps), and together serve as both source impedancefor the transmit function and as termination impedance for the receivefunction. In the described NM state, the modem can perform normaltransmit and receive functions.

When both the relays are released, as indicated in the table above, thetransceiver will be in the High Impedance State. The line, indicated byconnectors 9A and 4A, is connected to the probing transformer throughthe Relay KI and further to the line receiver through Relay K2 in the HIstate. The line transformer and the line driver along with resistors R28and R48 are isolated from the line through relay K1 and from the linereceiver through relay K2. The impedance seen by the line is theparallel combination of resistor R55 and the reflection of the value ofresistor R85 by the turns ratio (=1) of the probing transformer. Thevalues of R55 and R85 are much higher in value than the sum of R28 andR48 (about 25 times), therefore, the impedance seen by the line is veryhigh (about 10 times) with respect to the characteristic or normal lineimpedance, which is about 100 Ohms.

The capacitors perform the filtering function for signals in thespectrum below the ADSL band and can therefore be ignored in thisanalysis. As a result, the modem transceiver of the detailedimplementation presents a high impedance over the ADSL band offrequencies to the line and can still receive the signals on the linewithout disrupting them. The switches described in the four embodimentsabove can be realized in different ways. For example, they could be anyone, or a combination of more than one of the following: (1) Mechanical,(2) Electro-mechanical, (3) Electronic, or (4) Filter. They couldfurther be controlled manually, on command, or automatically, throughhardware and/or software. Further, as those skilled in the art willrecognize, although the discussion above has focussed on DSLtransceivers, and ADSL transceivers in particular, the discussion isapplicable to other high speed communication devices. For example, thediscussion is applicable to other high speed communication transceivers,such as co-axial modems, VDSL modems, and high speed communicationtransmitters. In addition, as those skilled in the art will recognize,the transceivers can be connected to any line of sufficient length to becharacterized as a transmission line.

From the foregoing it will be observed that numerous modifications andvariations can be effectuated without departing from the true spirit andscope of the novel concepts of the invention. It is to be understoodthat no limitation with respect to the specific embodiments and methodsillustrated is intended or should be inferred. The disclosure isintended to cover by the appended claims all such modifications as fallwithin the scope of the claims.

1. A high speed communication transceiver for use as one of a pluralityof high speed communication transceivers connected to a transmissionline, the transmission line having a normal impedance, the high speedcommunication transceiver comprising: a transmission line interfaceselectively coupled to the transmission line; a first receiver connectedto the transmission line interface; a second receiver selectivelycoupled to the transmission line; a transmitter connected to thetransmission line interface, the transmitter having an impedancesubstantially equal to the normal line impedance; and an analog frontend alternatively coupled to the first and second receivers, and whereinthe high speed communication transceiver presents a high impedance tothe transmission line with respect to the normal impedance of thetransmission line and the second receiver is coupled to the transmissionline and the analog front end when the transmission line interface isnot coupled to the transmission line, wherein the high speedcommunication transceiver presents an impedance to the transmission linesubstantially equal to the normal impedance of the transmission line andthe second receiver is not connected to the transmission line or analogfront end when the transmission line interface is coupled to thetransmission line and the analog front end is instead coupled to thefirst receiver.
 2. The high speed communication transceiver of claim 1wherein the second receiver is coupled to the transmission line by aprotection circuit and a first switch when the second receiver iscoupled to the transmission line.
 3. The high speed communicationtransceiver of claim 2 wherein the first switch includes mechanicalcontacts.
 4. The high speed communication transceiver of claim 2 whereinthe first switch includes electro-mechanical relays.
 5. The high speedcommunication transceiver of claim 2 wherein the first switch is anelectronic switch.
 6. The high speed communication transceiver of claim2 wherein the first switch is a filter.
 7. The high speed communicationtransceiver of claim 2 wherein the first switch is manuallycontrollable.
 8. The high speed communication transceiver of claim 2wherein the first switch is automatically controllable by hardware. 9.The high speed communication transceiver of claim 2 wherein the firstswitch is automatically controllable by software.
 10. The high speedcommunication transceiver of claim 2 wherein the transmission lineinterface is a hybrid circuit.
 11. The high speed communicationtransceiver of claim 2 wherein the transmission line includes bridgetaps.
 12. A high speed communication system comprising: at least onecentral office high speed communication transceiver; a transmission linehaving a normal impedance, wherein the at least one central office highspeed communication transceiver is connected to a first end of thetransmission line; a plurality of remote high speed communicationtransceivers connected to a second end of the transmission line, eachremote high speed communication transceiver including a first receiver,a transmission line interface, and a transmitter selectively coupled tothe transmission line, wherein each remote high speed communicationtransceiver presents a high impedance to the transmission line withrespect to the normal line impedance when the transmitter is not coupledto the transmission line and each remote high speed communicationtransceiver presents an impedance substantially equal to the normal lineimpedance to the transmission line when the transmitter is coupled tothe transmission line, wherein the transmitter is selectively coupled tothe transmission line by a first switch and the transmission lineinterface, and wherein the first receiver is selectively coupled to thetransmission line interface by a second switch and the transmission lineinterface is selectively coupled to the transmission line by the firstswitch, each remote high speed communication transceiver presenting ahigh impedance to the transmission line with respect to the normal lineimpedance and the first receiver being coupled to the transmission lineby the second switch and the first switch when the transmitter is notcoupled to the transmission line, and each remote high speedcommunication transceiver presenting an impedance to the transmissionline substantially equal to the normal line impedance and the firstreceiver being coupled to the transmission line interface by the secondswitch when the transmitter is coupled to the transmission line.
 13. Thehigh speed communication system of claim 12 wherein the transmitter iscoupled to the transmission line interface by the first switch and thetransmission line interface is connected to the transmission line. 14.The high speed communication system of claim 13 wherein the first switchis positioned within the transmitter.
 15. The high speed communicationsystem of claim 12 further including a protection circuit, wherein thefirst receiver is coupled to the transmission line by the second switch,the protection circuit, and the first switch when the transmitter is notcoupled to the transmission line.
 16. The high speed communicationsystem of 12 wherein each remote high speed communication transceiverfurther includes a second receiver selectively coupled to thetransmission line by the first switch, each remote high speedcommunication transceiver presenting a high impedance to thetransmission line with respect to the normal line impedance and thesecond receiver being coupled to the transmission line by the firstswitch when the transmitter is not coupled to the transmission line, andeach remote high speed communication transceiver presenting an impedanceto the transmission line substantially equal to the normal lineimpedance and the first receiver being coupled to the transmission lineby the transmission line interface and the first switch when thetransmitter is coupled to the transmission line.
 17. The high speedcommunication system of claim 16 wherein each remote high speedcommunication transceiver further includes a protection circuit, thesecond receiver being coupled to the transmission line by the protectioncircuit and the first switch when the transmitter is not coupled to thetransmission line.